Interleukin-1.beta. (IL-1.beta.) is a major mediator of chronic and acute intimation. Along with IL-1.beta., human monocytes produce two additional members of the IL-1 gene family; interleukin-1.alpha. (IL-1.alpha.) and IL-1 receptor antagonist (IL-RA). All three proteins bind to the membrane-anchored forms of the type 1 and type 2 IL-1 receptors (IL1R) on target cells. IL-1.alpha. and IL-1.beta. elicit virtually identical biological responses whereas IL-1RA blocks these effects. Both IL-1.alpha. and IL1.beta. are synthesized as 31 kDa primary translation products which lack functional hydrophobic signal sequences. The 31 kDa form of IL-1.alpha. is fully active without further processing but does not appear to be actively released from cells. IL-1.beta., the predominant form of IL-1 released by activated monocytes, is synthesized as an inactive 31 kDa precursor (pIL-1.beta.) that is processed to its mature 17.5 kDa form (mIL-1.beta.) by interleukin-1.beta. converting enzyme (ICE), a novel cysteine proteinase. ICE generates fully active mIL-1.beta. by cleaving pIL-1.beta. between Asp.sub.116 and Ala.sub.117, a unique site for prohormone processing. The sequence around this cleavage site, -Tyr-Val-His-Asp-Ala-, is evolutionarily conserved in all known pIL-1.beta. polypeptides.
Active human ICE as shown by conventional HPLC and affinity purification techniques is a heterodimer consisting of a 1:1 stoichiometric complex of 19,866 Da (p20) and 10,244 Da (p10) subunits. Cloned cDNAs have revealed that ICE is constituitively expressed as a 45 kDa proenzyme (p45) composed of a 14 kDa prodomain, followed by p20 which contains the active site Cys.sub.285, a 19 residue connecting peptide that is not present in the mature enzyme, and p10, a required component of the active enzyme. The mature subunits are flanked by Asp-X sequences. Mutational analysis of these sites and expression in heterologous systems indicates that the generation of active enzyme is autocatalytic. Murine and rat ICE have also been cloned and show a high degree of sequence similarity including these structural motifs.
Recently, a family of ICE-like genes has begun to emerge, including the nematode cell death abnormal gene (CED-3) of Caenorhabiditis elegans, Caenorhabiditis briggsae and Caenorhabiditis vulgaris, and the murine neuronal precursor cell embroyonic developmentally downregulated (NEDD-2) gene. The predicted polypeptide sequences of these genes exhibit 29% and 27% sequence identity with human ICE, respectively. The sequence identity of CED-3 with ICE is higher in the regions corresponding to the p20 and p10 subunits of mature human ICE. All known sequences for ICE and for CED-3 contain the pentapeptide sequence -Gln-Ala-Cys-Arg-Gly-surrounding the catalytic cysteine of ICE or its equivalent in CED-3.
Both CED-3 and murine ICE, when expressed by transfection in fibroblast cell lines or by microinjection into neuronal cells, cause programmed cell death (apoptosis) to occur. The pro-apoptotic effects of CED-3 or ICE can be prevented by co-transfection with either bcl-2, a mammalian proto-oncogene which appears to function as a cell death suppressor gene, or with the cytokine response modifier A (crmA) gene product, a serpin-like inhibitor of ICE.